When satellite telecommunications terminals are operated in proximity to radars (for example, shipboard satellite terminals operating in proximity to radars on or nearby the ship), interruptions and outages can occur at the satellite terminal when the radar is in use, even when the radar is not operating at the same frequency as the satellite signal. FIG. 1 depicts a scenario in which a shipboard satellite terminal 1 receives communication signals from a satellite 2, which relays information from a land earth station 3. An onboard navigation radar 4, which is located in proximity to the satellite terminal 1, will disrupt the functioning of the satellite terminal 1 when the navigation radar 4 is in use.
Depending on weather, location and function, radars may be in operation any time of the day or night, so interference could come from various sources: nearby ships, the same ship on which the satellite terminal is located, airport radars, or airborne early warning systems. Interference could occur even if the satellite terminal were separated from the radar source by several miles. The ability to create a telephone service business which is salable to the public and which meets international standards for availability requires that a means be found for maintaining high communications quality while radars are in use.
Current practice is to avoid operational use of the satellite terminal and nearby radar simultaneously, if possible, because there has been no effective way to use both the satellite terminal and radar simultaneously with the assurance that the telephone service will be maintained at a high level of quality. Disruptions are common when radar equipment is turned on.
FIG. 2 depicts the relevant parts of a conventional end-to-end satellite link. A land station 210 interfaces with terrestrial networks, encodes signals and transmits them to a satellite 220, which relays the signals to a mobile satellite terminal 230, which decodes the signals. The land station 210 includes a transmit modem 211 with forward error correction (FEC), an up-converter 212 to convert the modulated signal from the modem baseband to radio frequency (RF), a high power amplifier 213, an antenna 215, and a diplexer 214 with a bandpass filter to allow the antenna 215 to be used for both transmitting and receiving.
The satellite terminal 230 (or "receive terminal") has an antenna 231, a diplexer 232 with a bandpass filter, a low noise amplifier 233, a down-converter 234 to convert the RF signal to modem baseband, and a receive modem 235 with FEC. Components 231-235 make up the "front-end" of the satellite terminal.
In conventional systems where radar equipment and a satellite terminal are mounted in proximity, e.g., both on the same ship, solutions to the interference problem have been attempted but have not been completely successful. One reason for this may be that, to the inventors' knowledge, no one in the field has taken a serious look at the failure mechanism of communication links with nearby radars.
In a study performed for COMSAT Mobile Communications, it was found that, for L-Band satellite communications on U.S. Navy ships with high powered radars, the failure mechanism related to radar spurious signal energy entering the front-end of the satellite terminal. FIG. 3(a) shows a spectrum and waveform of a radar signal, offset from the band of a satellite receive signal. FIG. 3(b) shows a pulsed radar RF signal in the time domain. S.sub.O is the peak spectral level of the radar signal, .tau. is the pulse width of the radar signal, and T is the pulse repetition period. The radar's center frequency 310 is displaced from the satellite front-end frequency band 311 by tens or hundreds of megahertz. Even though spurious emission 312 in the far sideband of the radar may be very low in comparison to the main-band radar signal 313 (typically 50-100 dB down), it is still so strong at small distances that it interferes with the incoming satellite signal.
The data from the COMSAT study can be used to demonstrate, by computation, the various levels of interfering signals that are present in a C-Band terminal on a ship in proximity to a navigation radar. As a baseline, the scenario in Project CHALLENGE ATHENA: A Demonstration of Megabit Data Transmission to U.S. Naval Vessels, Classified Session 23, Paper #2, Milcom '93, Bedford, Mass. (CHALLENGE ATHENA), is used to generate representative numbers. Many variations on this scenario are possible. More powerful satellites will have potentially stronger signal components. Commercial navigation radars may not be as powerful as the Navy's AN/SPS-48, for example. The main conclusions stay the same, however. In order to better understand the power and dynamic range relationship among the various signals, an analysis was conducted to independently estimate the relative levels of wanted and unwanted signals within the configuration of the ship's earth station as described in CHALLENGE ATHENA.
Following the scenario from CHALLENGE ATHENA, it is assumed that a Navy carrier stationed between Norfolk, Va. and the eastern Caribbean is receiving a wideband 1.544 Mbps signal from an INTELSAT V (F6) satellite located over the Atlantic Ocean, at 310.degree. E. The satellite is transmitting the signal via a global antenna. The satellite operating effective isotropic radiated power (EIRP) is taken as 23.5 dBW. The elevation angle from the ship terminal to the satellite was calculated to be 45.6.degree., and the path loss at 4.15 GHz was calculated to be 196.3 dB. Table 1 summarizes the ship's terminal receive characteristics.
Link analysis was conducted to determine the received signal power and noise power (N) at the output of the terminal's antenna flange. Noise power levels were calculated using N=kTB for full terminal bandwidth (500 MHz), transponder bandwidth (40 MHz) and signal bandwidth (1.85 MHz). The results are summarized in Table 2.
TABLE 1 ______________________________________ Terminal Receive Characteristics PARAMETER VALUE ______________________________________ Terminal (INTELSAT Std G) STD G Radome Diameter (m) 2.74 Radome Loss (dB) 0.5 Radome Noise temperature (K..degree.) 34 Antenna Diameter (m) 2.2 Antenna gain (dBi) @ 4.15 GHz 37.5 Antenna Sidelobe Gain (dBi) G(.phi.) = 32 - 25log(.phi.) Antenna Noise temperature (K..degree.) 25 LNB Noise temperature (K..degree.) 80 System Noise Temperature (K..degree.) 139 Terminal G/T (dB/K) 15.6 Terminal Bandwidth (MHz) 500 ______________________________________
TABLE 2 ______________________________________ Received Signal and Noise Analysis PARAMETER VALUE ______________________________________ Satellite Saturated EIRP (dBW) 25.5 Output Backoff (dB) 2.0 Satellite Operating EIRP (dBW) 23.5 Path Loss (dB) @ 4.15 GHZ 196.3 Pointing Error (dB) 1.0 Terminal Antenna Gain (dBi) 37.5 Terminal Radome Loss (dB) 0.5 Received Signal Strength (dBm) -106.8 System Noise Temperature (dBK..degree.) 21.4 Boltzmann's Constant -228.6 Noise Power Density (dBm/Hz) -177.2 Signal Bandwidth, 1.85 MHz (dBHz) 62.7 Noise Power in Signal Bandwidth (dBm) -114.5 Transponder Bandwidth, 40 MHz (dBHz) 76.0 Noise Power in Transponder Bandwidth -101.2 (dBm) Terminal Bandwidth, 500 MHz (dBHz) 87.0 Noise Power in Terminal Bandwidth -90.2 (dBm) ______________________________________
The RF interference source to the STD G terminal, located on a sponson (a small flat structure that projects over the side of a ship) about 100 meters away from the Carrier's mast, was the AN/SPS-48C surveillance radar. The AN/SPS-48 is a three dimensional, long range air surveillance radar. It uses a combination of mechanical scanning in azimuth and electronic beam steering in elevation to provide position and altitude information. Table 3 summarizes the AN/SPS-48 performance characteristics.
TABLE 3 ______________________________________ AN/SPS-48 Performance Characteristics ITEM PARAMETER VALUE ______________________________________ Antenna Size 17 .times. 18 Beamwidth (deg) 1.6.degree. .times. 1.6.degree. Gain (dBi) 39 Transmit Power Peak (MW) 2.2 Average (kW) 35 Signal Frequency (GHz) 2.9-3.1 Pulse Width (.mu.sec) 9 and 27 PRF (Hz) 342-2778 ______________________________________
The methodology employed here is to use the AN/SPS-48 radar published characteristics along with a free space loss analysis to determine typical interference levels. Consider the case where an INTELSAT Std-G terminal is receiving an unwanted signal from a AN/SPS-48 radar located 100 m away. The assumption is that the interference signal is received from the radar's antenna sidelobe (assumed to be -40 dB) and enters the INTELSAT terminal via its own antenna sidelobe. The elevation angle from the terminal to the radar was calculated to be 11 degrees. For a worst case analysis, it is assumed that the radar and the satellite are at the same azimuth angle relative to the terminal.
The radar signal presents two interference components to the terminal. One such component results from spectral energy in the out-of-band emission region of the AN/SPS-48. More precisely, low levels of energy emitted by the AN/SPS-48 radar in its out-of-band region enter the INTELSAT terminal in its own receive band. Discussions with the manufacturer of the radar, and measured spectral data under laboratory conditions, suggest emission levels of 90 dB below peak. However, under operational conditions, in the field and in particular with older equipment, these levels could be 10 or 20 dB higher and still meet MIL-STD-469A requirements for electromagnetic compatibility. Gawthrop, P. E., and Patrick, G. M., Ground-Based Weather Radar Compatibility with Digital Radio-Relay Microwave Systems, U.S. Department of Commerce NTIA Report 90-260, March 1990, section 5 ("Gawthrop"), reports on spurious levels for commercial navigation radars in the 50 dB to 60 dB down range, at several hundred MHz separation.
The other interference component results from the AN/SPS-48 inband emission which enters the INTELSAT terminal outside the diplexer pass band. FIG. 3(a) illustrates the power spectrum of a pulsed radar. The center frequency for the AN/SPS-48 is 2.96 GHz and its signal bandwidth is on the order of 37 kHz. The center frequency of the INTELSAT G terminal is at 3.95 GHz. Given that the INTELSAT G terminal employs a diplexer whose out-of-band rejection is 70 dB, any signal emitted by the radar in its own central band will be received by the satellite terminal 70 dB down.
Referring to FIG. 3(a), the peak spectral level (So) is the product of the average transmit power (P.sub.av) and the pulse width (.tau.). If the desired peak spectral level is in dB, then the corresponding product is achieved by assigning a dB value to the average transmit power and pulse width as seen in the following equation. EQU S.sub.O =P.sub.av +.tau.,
At the input to the LNA 233 (FIG. 2), the interfering signal consists of two major components. One component, P.sub.int, is due to the radar's out-of-band emission in the terminal's diplexer 500 MHz pass-band (also in dB): EQU P.sub.int =S.sub.o -90+87
The other component, P.sub.radar, is due to the radar's in-band emission in the satellite terminal's stop-band region (-70 dB): EQU P.sub.radar =P.sub.av -70
The relative power level between the interference caused by the radar's out-of-band and its in-band signals is, ##EQU1##
This result shows that improvements to the diplexer will not help, since the noise entering the INTELSAT G terminal front end from the radar's out-of-band emission is about 21 dB larger than the pulses that pass through the diplexer stop band. As a consequence, it should be noted that, when RFI hits the terminal, the predominant effect at the LNA 233 is due to the radar's out-of-band emission and not the main pulses.
Thus, for the close-in distances between a ship's radar and its satellite terminal, the spurious emission entering the front-end is more powerful than the (diplexer-filtered) main band energy of the radar. The spurious emission is sufficiently strong, and the pulse duration is sufficiently long, that when a pulse is "ON" it will disrupt the convolutional decoder normally used with modems on satellite terminals. Once errors occur, the decoder may or may not be able to recover during the noise-free "OFF" period of the radar because error events and noise have been added to the digital bit stream during the ON condition.
This failure mechanism is illustrated in FIG. 4, which treats the radar ON and OFF periods as changes in the instantaneous carrier-to-noise ratio (C/No) that are seen by the convolutional decoder. This analogy is valid because it is well known in the art that the time waveform response of the satellite terminal to the radar pulse is a noise-like signal (incoherent) whose duration is the same as the radar pulse, as shown in FIG. 5. Note also that the satellite terminal's response sometimes includes an impulsive "click" at the start and stop of the radar pulse. See Gawthrop, supra.
FIG. 4 depicts the relative time periods of radar pulses .tau., pulse repetition period T, channel symbols S (encoded information bits), convolutional constraint lengths L and path metric memory M. FIG. 4 shows that, based on a typical satellite terminal bit rate of 1.544 Mbps, and a typical radar pulse duration .tau., ordinary Viterbi algorithm or sequential decoding algorithm convolutional decoders get "swamped" (overwhelmed) by the elevated noise level caused by the radar pulse. This occurs because .tau. lasts for several constraint lengths of the convolutional code in the case of the Viterbi algorithm, and a large fraction of a constraint length in the case of the sequential algorithm. .tau. may also exceed the path metric memory size M, which typically extends back about three constraint lengths.
Conventional solutions to this problem partially or inadequately address getting end-to-end performance to be consistently and reliably within stringent bit error rate (BER) conditions, regardless of the presence or absence of the radar signal.
One approach adds a diplexing filter with better cutoff characteristics to the diplexer 232 of the conventional satellite terminal 230 depicted in FIG. 2. However, this does not eliminate the in-band energy which is causing failure of the decoder, as discussed supra.
A second approach adds a pulse blanking circuit to the satellite terminal 230 that opens when the radar pulse is ON. However, if the blanker is on the antenna side of the diplexer 232, it would disrupt signals transmitted by the terminal (i.e., the return link to land). If the blanker is on the LNA side of the diplexer 232, it does not respond adequately to the radar spurious noise in the satellite band, because this noise has characteristics very different from the radar pulse shape it is trying to detect. FIG. 5 shows the shape of the radar spurious noise which comes through the satellite terminal's front end vs. the shape of the radar pulse. Furthermore, attempting to "sense" the radar pulse on the antenna side of the diplexer 232 and then controlling the blanker on the LNA side does not work because the received radar pulse is very weak compared to the signals transmitted by the terminal. Regardless of where the blanking circuit is placed, and even if it operates correctly to eliminate only the affected channel symbols, the duration of the ON pulse is still too long compared to the convolutional code's constraint length as shown in FIG. 4. Thus, a blanking operation, applied without any further modification of the system, produces periodic outages and a loss of information.
A third technique is to add interleaving and de-interleaving to the conventional satellite system. Interleaving the channel symbols at the transmit modem 211 at the land station 210 and de-interleaving them at the receive modem 235 disperses the channel symbols that are hit by the ON radar pulse, but still leaves elevated levels of effective noise power spectral density on the affected channel symbols. This is a problem because the soft-decision decoder in the receive modem 235 will then propagate the elevated noise within its metric calculations. Satellite digital carriers are normally operated close to threshold. Since the elevated noise from the spurious radar emission can be tens of dB higher than background thermal noise, the radar's duty factor, which may be only a few percent, is effectively multiplied by the difference in noise levels.
A fourth approach is to simply boost the satellite power per carrier as used on a current state of the art terminal. However, increasing the EIRP of the signal from the satellite to the satellite terminal may not be sufficient to overcome the elevated levels of noise during the ON state of the radar. Inordinate increases in EIRP, combined with decreases in the digital data rate, may provide a short-term "fix", in the sense that it is getting some information through, even if not at data rates that were originally intended, but this approach is very wasteful of the satellite resource. Additionally, it requires unusual operational changes like ad-hoc requests for more power, data rate changes, and traffic dumping.